Here's an even better (and much simpler) way to merge the fids directory from drive1 into that of drive0, regardless of fids subdirs or not:

cp -auvfx /drive1/fids/* /drive0/fids
## and then, optional step to nuke the old files:
rm -r /drive1/fids/*


Nice and simple. Just run fidsift.sh again aterwards.

Cheers